1. Field of the Invention
The present invention relates to a recording sheet, an ink-jet recording method using the sheet, and an electrophotographic image recording method using the sheet.
2. Description of the Related Art
An ink-jet recording system has characteristics such as easy colorization, reduced consumption of energy, low noise during recording and low manufacturing cost of a printer. Thus, wide attention is directed to this system. Further, improvements in image quality, high-speed operation and reliability have been made in recent years. Printing is frequently made on plain sheet and it is therefore very important to improve recording quality on plain sheet.
In conventional major ink-jet printers, black ink includes a pigment as a colorant and has low penetrability in recording sheets (hereinafter simply referred to as “sheet”) and color inks use dyes as colorants and have high penetrability in recording sheets. These inks are selected in order to improve image quality of black characters and prevent mixed color bleeding.
Therefore, when the color ink, which have high penetrability in sheets, are used to print an image with high recording density, heavy curl and cockle of a recording sheet occur immediately after printing, leading to jamming in a printer and the rubbing of the image part. Moreover, when duplex printing is conducted, the time required for improvement of the curl and the time required for drying of ink reduces print productivity extremely. Further, when an image having high recording density is printed, heavy curl and cockle occur after air drying of the image. It is therefore impossible to satisfy high quality, suppression of curl and cockle simultaneously at high levels.
The following methods have been proposed to prevent the aforementioned curl and cockle after printing: a method in which a sheet obtained by sheet-making is humidified once to lighten the stress of sheet, thereby reducing curl and cockle (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 3-38375), a method in which the in-water ductility of sheet in a CD is limited to reduce curl and cockle (see, for example, JP-A No. 3-38376), a method in which the ratio of the in-water ductility of sheet in a MD to the in-water ductility of sheet in a CD is designed to be 1.3 or less to reduce curl and cockle (see, for example, JP-A No. 3-199081), a method in which the in-water ductility of a ink-discharged part in a moving direction is designed to be 2.0% or less to reduce curl and cockle (see, for example, JP-A No. 7-276786), a method in which the in-water ductility in a CD is designed to be 1.8% or less to suppress curl and cockle (see, for example, JP-A No. 10-46498) and a method in which the amount of a pigment contained in a support is designed to be 5 to 35% by weight and the internal binding strength of a recording sheet (recording sheet) is designed to be 150 to 455 g/cm to suppress the cockle of a coat-type ink-jet recording sheet (see, for example, JP No. 3172298).
In these methods described in the aforementioned documents, it is reported that curl and cockle are suppressed. However, when ink having high penetrability into a recording sheet is used and a lot of ink is discharged or printing speed is so high that the amount of ink to be discharged per time is increased, heavy curl occurs and the sheet is impractical for document use.
Also, the method described in U.S. Pat. No. 3,172,298 attempts to reduce swelling after printing by limiting the internal binding strength of a recording sheet having an ink receiving layer within a specified rang. However, the limitation to the internal binding strength alone fails to obtain a sufficient effect on the prevention of curl, cockle and swelling. Particularly, when ink having high penetrability into a recording sheet is used and a lot of ink is discharged or printing speed is so high that the amount of ink to be discharged per time is increased, heavy curl occurs and the sheet is impractical for document use.
In order to suppress curl and cockle after air drying of a sheet, there is proposed a method in which the irreversible shrink ratios in a MD and in a CD when environmental relative humidity is changed, is made to fall within a specified range to thereby suppress curl and cockle after the drying (see, for example, JP No. 3127114). However, when there is no limitation on the penetration of ink into a recording sheet and ink having high penetrability into a recording sheet is used and a lot of ink is discharged, the ink penetrates into the inside of the recording sheet. As a result, an absolute amount of fibers which are to shrink after drying is increased as a whole, causing an increase in curl after the drying, leading to only insufficient effects.
On the other hand, a method is proposed in which an ester type nonionic surfactant having an HLB of 11 or more is compounded in an ink receiving layer to improve image quality (see, for example, JP-A No. 10-278409). However, this surfactant is highly hydrophilic and therefore, the hydrophilic groups of a base material (base sheet) cannot be coated with the hydrophobic groups of the surfactant. When a large amount of ink is used, a recording sheet is easily deformed, leading to heavy curl and the sheet is impractical as a document.
Also, a method is proposed in which a surfactant having an HLB ranging from 3 to 12 is compounded in an ink receiving layer on the film surface to improve image quality (see, for example, JP-A No. 62-144986). However, even if the above technologies are applied to plain sheet, the amount of the surfactant is as low as less than 0.1% by weight and therefore the hydrophilic groups of a base material cannot be coated with the hydrophobic groups of the surfactant. Particularly, when ink having high penetrability into a recording sheet is used and a lot of ink is discharged or printing speed is so high that the amount of ink to be discharged per time is increased, large cockle occurs and the sheet impractical as a document.
Also, a method is proposed in which dry oxidized starch is subjected to size press treatment for preventing curl (see, for example, JP-A No. 2002-348798). However, when a lot of ink is used for printing, the above technologies cause water in the ink to elongate a base material, thus heavy curl occurs and the sheet is impractical as a document.
Further, in order to improve image quality in an ink-jet system, an ink receiving layer containing a silanol modified polyvinyl alcohol (PVA), 11 to 20% by weight nonionic surfactant and a synthetic amorphous silica as a filler is proposed (see, for example, JP-A No. 11-115304). In this method, there is no teaching on the HLB of a surfactant and also, since the hydrophilicity of the surfactant is high whose example is the surfactant used is in the examples with the HLB of 14, the hydrophilic groups of the base material cannot be coated with the hydrophobic group of the surfactant when this method is applied to a sheet derived from cellulose pulp. The above technologies likewise allows water in the ink to elongate a base material, thus heavy curl occurs and the sheet is impractical as a document.
Also, a method in which a bulky-softening agent having a HLB of 6 or less is compounded in a print sheet is proposed to improve the bulkiness-softness. However, a surfactant having a HLB of 6 or less, particularly a surfactant having a HLB of 4 or less, has poor dispersibility. Consequently, the hydrophilic groups of a base material cannot be coated with the hydrophobic groups of the surfactant and the absolute amount of fibers which stretch and shrink increases, and heavy curl occurs. As a result, only insufficient effects are obtained (see, for example, JP-A No. 2002-155494).
Further, a method is proposed in which, in an electrophotographic system, water retention and freeness of the pulp obtained by disintegrating a recording sheet is adjusted within ranges of 80 to 110% and 480 to 600 ml respectively to improve reliability (see, for example, JP-A No. 9-119091). In this method, curl can be reduced by controlling the water retention value of pulp. However, sheet strength is reduced and there is therefore the case where sheet powder is easily produced in an electrophotographic printer and a copying machine, and low stiffness causes jamming. Also, when this method is applied to an ink-jet system, owing to vaporization of water in ink, heavy curl occurs at printing and drying.
Also, in an ink-jet system, a method is proposed (see, for example, JP-A No. 2002-201597) which aims at shape stabilization by impregnation with a cellulose cross-linking agent. In this method, however, if the degree of bonds between fibers becomes excessive by the cross-linking treatment, heavy curl occurs as water in ink vaporize when a sheet is dried after printing. Also, this technology cannot suppress deformation of a sheet immediately after printing and jamming is likely to occur in a machine. Moreover, when this sheet is applied to an electrophotographic copying machine and printer, residual formaldehyde is vaporized in a fixing unit and there is a fear that not only the machine is contaminated but also the vapor is harmful to people who touch the sheet.